Recently, techniques for making a contactless measurement of minute displacements of an object using, as an index, a pattern or texture having a spatially random structure such as a laser speckle pattern and a random dot pattern, play an important role in industrial applications including non-destructive inspections and material strength inspections. The correlation technique is known as a signal processing technique that is used when detecting minute displacements using the pattern or texture.
FIGS. 38 and 39 are diagrams for explaining an operation of the correlation technique. FIG. 38(A) shows displacements in a field, FIG. 38(B) shows the concept of the correlation technique, and FIG. 38(C) shows the concept of the measuring technique. In addition, FIG. 39(A) is a diagram for explaining a parallel displacement of an object A in one direction, and FIG. 39(B) is a diagram for explaining a rotary displacement of the object A.
When detecting the displacement by using the correlation technique, if the displacements in the field differ depending on the position as indicated by arrows in FIG. 38(A), it is necessary to separately compute the displacement for each region as shown in FIG. 38(B), and it is necessary to compute a two-dimensional correlation integration or a two-dimensional Fourier transform or inverse transform every time the correlation value is determined at each point. For this reason, if the number of computation points of the correlation value within the pixel is increased in order to increase the resolution of the displacement detection in one pixel or less, the computation time becomes extremely long.
In addition, according to the conventional correlation technique, it is possible to obtain the amount of displacement by obtaining the correlation with respect to the displacement in one direction as shown in FIG. 39(A), but this could not be applied to the rotary displacement as shown in FIG. 39(B).
As a method of computing the correlation function using phase information, the phase-only correlation technique is known (refer to Patent Document 1). This correlation technique suppresses the amplitude of a synthesized spectrum, which is a function of complex variable, to a constant or by use of a logarithmic function or the like, in order to create a fixed-amplitude complex synthesis spectrum solely including phase information and to obtain a cross-correlation function by subjecting the fixed-amplitude complex synthesis spectrum to a inverse Fourier transform.
Patent Document 1: Japanese Patent No. 3035654